Leviathans are an ancient race of gigantic, aquatic, sentient cephalopods that ruled a significant portion of the galaxy during the age of the forerunners. They vaguely resemble Earth's squids, and their body is covered in bony, rigid plates. They also have the unique ability to manipulate certain regions of other species' brains, giving them a sort of mind-control ability. They evolved on the planet Terinsha'ek, in the outer rim of the Milky Way Galaxy. The leviathans are the original creators of the Reapers.
During ancient times, the leviathans and forerunners had a very tense relationship. The leviathans were attempting to expand their territory, but their intentions conflicted with those of the forerunners. This conflict erupted into a brief war around 600,000 B.M. This war coincided with Chaos' invasion of Existence, subsequently causing a civil war among the Forerunner civilization. As a result, the war only lasted half a decade.
The status of the leviathan race is currently unknown, however, it is believed that they still exist today, somewhere far away amongst the stars. The most recent sign of their existence was shortly before the War of Deceit during the 2160s C.E. It was then that they pledged a large number of their Reaper machinations to the Dark Gods for their purposes, in exchange for the continued leviathan rule in their sector of space. Otherwise, the leviathans have not made their existence known to anyone.